This invention relates to the curing of a halogenated polyethylene with sulfur and/or a sulfur donor and a specified amino acid, and to the curing of a composition comprising a halogenated polyethylene, a specified sulfur-curable rubber and/or a vinyl chloride resin with sulfur and/or a sulfur donor and a specified amino acid.
Halogenated polyethylenes are known polymers prepared by halogenating polyethylene which have superior ozone resistance, weatherability, fire retardancy, low flex cracking, etc. It is known that halogenated polyethylenes are cured with suitable curing agents for use as rubbery elastomers. Chlorinated polyethylene is the most widely used halogenated polyethylene.
Typical known methods for curing halogenated polyethylenes include the use of organic perioxides such as dicumyl perioxide as a curing agent, and the use of a mixed curing agent composed of a metal oxide such as magnesium oxide and an organic compound known as a curing accelerator for rubber, such as ethylenethiourea or tetramethylthiuram disulfide. These methods, however, have one or more defects. In the case of curing with organic perioxides, open steam cure cannot be employed because it will cause surface tackiness, and the method of curing is limited to a press cure process. The latter method has the defect that because the metal oxide remains in the cured rubber product and may dissolve out during use of the product to cause health hazards, the cured rubber product cannot be used as rubber articles for medicines or foodstuffs (for example, stoppers for bottles). Furthermore, during the processing of rubber, the noxious metal oxide particles scatter about to affect the health of the working personnel.
Since halogenated polyethylenes have good miscibility with general rubbers such as natural rubber or SBR, curing of a blend of a halogenated polyethylene and a general rubber has been suggested. A typical method for curing such a blend involves the use of an organic peroxide which is a curing agent both for the halogenated polyethylene and the general rubber. As previously stated, this method has the defect that the open steam cure process cannot be employed. Another method for curing this blend comprises using separate curing systems for the halogenated polyethylene and the general rubber, and curing the blend by using these two curing systems simultaneously. A typical example of such a mixed curing system is composed of a curing agent comprising magnesium oxide and ethylenethiourea for curing the halogenated polyethylene, and sulfur and/or a sulfur donor, a vulcanization accelerator and a vulcanization activator for curing the general rubber. In order to obtain satisfactory curing of the general rubber, it is usually desirable to employ zinc oxide as the vulcanization activator. Zinc oxide, however, has a degenerative action on the halogenated polyethylene.
The present inventor has now found that a halogenated polyethylene can be well cured with a combination of sulfur and/or a sulfur donor and a specified amino acid. It has also been found that a composition composed of a halogenated polyethylene, a specified sulfur-curable rubber and/or a vinyl chloride resin can be cured well with a novel curing system composed of sulfur and/or a sulfur donor and a specified amino acid. The novel curing system has the advantage that it permits the application of both open steam cure and press cure, and does not require the use of a vulcanization activator such as zinc oxide.